First, the LTE system is a mobile communication system that has evolved from a UMTS system, and the standard has been established by 3rd Generation Partnership Project (3GPP), which is an international standardization organization.
FIG. 1 is a view illustrating the network architecture of an LTE system, which is a mobile communication system to which the related art and the present invention are applied.
As illustrated in FIG. 1, the LTE system architecture can be roughly classified into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC). The E-UTRAN may include a user equipment (UE) and an Evolved NodeB (eNB, base station), wherein the connection between UE-eNB is called a Uu interface, and the connection between eNB-eNB is called an X2 interface. The EPC may include a Mobility Management Entity (MME) performing a control-plane function and a Serving Gateway (S-GW) performing a user-plane function, wherein the connection between eNB-MME is called an S1-MME interface, and the connection between eNB-S-GW is called an S1-U interface, and both connections may be commonly called an S1 interface.
A radio interface protocol is defined in the Uu interface which is a radio section, wherein the radio interface protocol is horizontally comprised of a physical layer, a data link layer, a network layer, and vertically classified into a user plane (U-plane) for user data transmission and a control plane (C-plane) for signaling transfer. Such a radio interface protocol can be typically classified into L1 (first layer) including a PHY layer which is a physical layer, L2 (second layer) including MAC/RLC/PDCP layers, and L3 (third layer) including a RRC layer as illustrated in FIGS. 2 and 3. Those layers exist as a pair in the UE and E-UTRAN, thereby performing data transmission of the Uu interface.
FIGS. 2 and 3 are exemplary views illustrating the control plane and user plane architecture of a radio interface protocol between UE and E-UTRAN in an LTE system, which is a mobile communication system to which the related art and the present invention are applied.
The physical layer (PHY) which is a first layer provides information transfer services to the upper layers using a physical channel. The PHY layer is connected to the upper Medium Access Control (MAC) layer through a transport channel, and data between the MAC layer and the PHY layer is transferred through the transport channel. At this time, the transport channel is roughly divided into a dedicated transport channel and a common transport channel based on whether or not the channel is shared. Furthermore, data is transferred between different PHY layers, i.e., between PHY layers at the transmitter and receiver sides.
Various layers exist in the second layer. First, the Medium Access Control (MAC) layer serves to map various logical channels to various transport channels, and also performs a logical channel multiplexing for mapping several logical channels to one transport channel. The MAC layer is connected to an upper Radio Link Control (RLC) layer through a logical channel, and the logical channel is roughly divided into a control channel for transmitting control plane information and a traffic channel for transmitting user plane information according to the type of information to be transmitted.
The Radio Link Control (RLC) layer of the second layer manages segmentation and concatenation of data received from an upper layer to appropriately adjust a data size such that a lower layer can send data to a radio section. Also, the RLC layer provides three operation modes such as a transparent mode (TM), an un-acknowledged mode (UM) and an acknowledged mode (AM) so as to guarantee various quality of services (QoS) required by each radio bearer (RB). In particular, AM RLC performs a retransmission function through an automatic repeat and request (ARQ) function for reliable data transmission.
A Packet Data Convergence Protocol (PDCP) layer of the second layer performs a header compression function for reducing the size of an IP packet header which is relatively large in size and contains unnecessary control information to efficiently transmit IP packets, such as IPv4 or IPv6, over a radio section with a relatively small bandwidth. Due to this, information only required from the header portion of data is transmitted, thereby serving to increase the transmission efficiency of the radio section. In addition, in the LTE system, the PDCP layer performs a security function, which includes ciphering for preventing the third person's data wiretapping and integrity protection for preventing the third person's data manipulation.
A radio resource control (RRC) layer located at the uppermost portion of the third layer is only defined in the control plane. The RRC layer performs a role of controlling logical channels, transport channels and physical channels in relation to configuration, re-configuration, and release of Radio Bearers (RBs). Here, the RB denotes a logical path provided by the first and the second layers for transferring data between the UE and the UTRAN. In general, the establishment of the RB refers to a process of stipulating the characteristics of protocol layers and channels required for providing a specific service, and setting each of the detailed parameter and operation method thereof. The RB is divided into a signaling RB (SRB) and a data RB (DRB), wherein the SRB is used as a path for transmitting RRC messages in the C-plane while the DRB is used as a path for transmitting user data in the U-plane.
In general, when there is data to be transmitted to the UE, the base station transmits the data to the UE, and then waits receipt notification from the UE. If it is notified that the UE has successfully received data, then the base station deletes the data from its own buffer. However, if it is notified that the UE has not successfully received data, then the base station retransmits the data.
However, it is difficult to make a direct data transmission and reception between the base station and the UE due to the introduction of a relay node (RN). In other words, even if the base station has successfully transferred the data to the relay node, the base station is unable to know whether or not the data has been successfully transferred from the relay node to the UE. Similarly, from a standpoint of the UE, even if the UE has successfully transferred the data to the relay node, the UE is unable to know whether or not the data block has successfully transferred from the relay node to the base station.
Typically, each UE has a possibility of continuously moving its location over a mobile communication system. For example, if data blocks 1, 2, and 3 have been successfully transferred from the base station to the relay node, then the base station can delete the data blocks 1, 2, and 3 from its own buffer. Then, the relay node will start to transmit the data blocks 1, 2, and 3 to the UE. In this situation, it may happen that the UE moves to a new area while the relay node transmits the data block to the UE. In case where the UE is out of the connected relay node, there may be a problem of occurring a data block that cannot be successfully transferred to the UE among the data blocks received from the base station to the relay node.
Furthermore, every base station (eNB) does not support RN. For example, Rel-8 eNB does not support RN. In this case, the RN should not attempt an access to a Donor eNB (DeNB). If the RN is accessed to an eNB that does not support RN, data generated by the RN itself may be processed to be transferred by the eNB but data generated by the UE that has been accessed to the RN cannot be processed by the eNB. In this case, the UE that has been accessed to the RN merely consumes the radio resources and battery, but is unable to receive any services.
In addition, if an additional access of the UE to the RN occurs when an amount of radio resources allocated to the RN is limited over an Un interface, then a radio allocation amount to the UE is limited, thereby causing a problem that the UE cannot properly receive services or another UE cannot properly receive services if the priority of the UE is higher. As a result, the RN cannot allow an access from the UEs with no particular plan in view, thereby requiring a suitable control method for managing this.